Photoacoustic detection uses piezoelectric transducers to measure pressure waves from the radiationless decay of a laser excited state, generated by pulsed irradiation. The conversion of energy to heat generates a pressure wave that can be detected with a piezoelectric transducer. Presently, photoacoustic detection is a very sensitive technique but it is not very selective.
Traditionally, two types of dual element transducers are commercially available. First, a dual element longitudinal transducer in a single housing wherein both elements are matched to have the same frequency, for example 1 MHz and 1 MHz, or 5 MHz and 5 MHz, have been used for thickness gauging and/or flaw characterization. One element is used for transmitting a signal and the second is used for receiving the returned signal. The two matched elements are angled toward one another to create a crossed beam sound path to yield better near surface resolution. In making these dual element longitudinal transducers, the frequencies are carefully matched in order to assure an optimum transmitter/receiver beam overlap. The second type of commercially available photoacoustic transducer is a dual element longitudinal/shear wave transducer in a single housing. This is used to make both longitudinal and shear wave measurements with a single transducer. For example a longitudinal element may have a frequency of 1 MHz and the shear element may have a frequency of 2.5 MHz. Each element is specific for either shear waves or longitudinal waves. This second type is used for specific applications where the transducer may be fixed in order to obtain both the longitudinal and shear wave signals. Couplants used for acoustic scanning do not transmit shear wave signals.
In both types of transducers, the amplitude of the observed photoacoustic signal provides the sensitivity, i.e. measurement of the quantity of absorbing species. However, the selectivity, i.e. identifying the absorbing species in a mixture may be possible by seeking one or more wavelengths that is/are not absorbed by certain species of the mixture. Because of overlap of absorption spectra, this technique is limited to mixtures of species with limited absorption spectra overlap.
Thus, there is a need in the art for a photoacoustic transducer that provides greater selectivity without relying upon limited absorption spectra overlap.